Exposure-age dating of rock avalanche debris at Raven Crag (Pasture Beck valley, Hartsop) using cosmogenic 10Be

Peter Wilson, Derek Fabel

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    Abstract

    A recent paper (Wilson & Jarman, 2022) documented 84 rock slope failures (RSFs) in the Lake District and the adjacent Howgill Fells. The definition adopted for a RSF was that used by Jarman & Harrison (2019, p.202):
    any substantial rockmass exposed to slope gravitational processes which has lost structural integrity, regardless of its degree of disintegration or distance travelled.
    A minimum threshold size of 0.01 km2 (100x100 m) was applied by Wilson & Jarman (2022) in order to exclude small-scale rockfalls, which are abundant in the fells of Lakeland. To determine RSF size, source ‘cavity’, travel path and debris accumulation areas were combined. Although a tripartite system of RSF classification was employed - i.e. (1) rock slope deformations of both extensional and compressional modes, (2) rockslides of typically arrested translational form, and (3) rock avalanches of cataclasmic and sub-cataclasmic types - in reality RSFs may be complex in that they can display characteristics of more than one of these categories. Thus in some circumstances classification may be difficult.
    In the inventory compiled by Wilson & Jarman (2022) 12 RSFs were assigned to the sub-cataclasmic rock avalanche category. These comprise failures that have largely evacuated their source areas and have disintegrated but the leading edge of the debris has only just reached the slopefoot, with the bulk of the debris aligned along the travel path. Failures of this type are invariably at the lower end of the RSF size range (0.01-0.3 km2 in the Lake District).
    Age assessment of Lake District RSFs has so far relied on a consideration of their morphology along with evaluation of their relationship to Late Devensian (32-11.7 ka) glacier margins. This has resulted in a small number of failures being inferred to pre-date Late Devensian glaciation. These failures, or parts thereof, are likely to have been reactivated during or following ice wastage. A few other failures are of undoubted Holocene (11.7 ka-present day) age because they either occurred in recent centuries or they occupy locations that were buried beneath glacier ice during restricted glaciation of the Younger Dryas Stadial (12.9-11.7 ka). Therefore the majority of the failures are likely to have occurred during wastage of the Lake District’s Late Devensian ice dome (~20-14.7 ka) and/or in the Lateglacial period (14.7-11.7 ka). However, numerical ages have not yet been obtained for any of the 84 documented failures.
    In this paper we report two cosmogenic nuclide (10Be) ages for boulders from the debris of the sub-cataclasmic rock avalanche at Raven Crag (Pasture Beck valley, Hartsop). These are the first numerical ages to be reported for a Lake District RSF, although cosmogenic ages for RSFs in other mountain areas of Britain have been available for several years. The rock avalanche debris is located within Younger Dryas ice limits and it displays a downslope rather than down-valley configuration – i.e. debris alignment does not indicate any overriding and down-valley transport by glacier ice. Together these factors support a Holocene age for the rock avalanche.
    Original languageEnglish
    Pages (from-to)31-35
    Number of pages5
    JournalThe Cumberland Geologist
    Volume3
    Publication statusPublished (in print/issue) - 10 Mar 2023

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